2003 Volume 52 Issue 4 Pages 218-222
Field studies on the AC corrosion risk were conducted on a steel pipeline newly buried in proximity to an elevated AC powered rail transit system. The pipeline is coated with extruded polyethylene coatings and cathodically protected using two CP rectifiers. AC corrosion risk was assessed with respect to the cathodic protection criteria that have been developed by the authors based on AC and DC current densities measured using coupons. The average of AC current density of each coupon connected to the surveyed pipeline satisfied the criteria at all the test stations in the test section where distributed grounding system using distributed magnesium electrodes was installed for the mitigation of induced AC. However, AC current density reached at considerably high levels for short time periods during the passing of AC powered trains where the pipeline parallels the AC powered rail transit system. Then, the pipe was connected to a bare steel casing pipe through a solid-state DC decoupling device for AC mitigation. Consequently, AC current density decreased to a low level even while AC powered trains were passing and the risk of AC corrosion was completely removed.
